To improve the sound-muffling property of a discharge pipe system made with plastic pipe segments, DE-OS [German Specification] 27 14 576 suggests providing the outside of the pipe components with a sound-muffling layer of an open-pore, soft plastic foam, with an inorganic filler. Soft polyurethane foam is suggested for the plastic foam and calcium carbonate or heavy spar for the filler. A system constructed with such pipes has the problem of difficult repairs, particularly since butt welding--frequently used in repairs--is hardly possible. There is always a circular lap all around the outside of the sleeve ends and the thickness of the sound-muffling layer basically corresponds to the external diameter of the lap which surrounds the outside of the sleeve ends. To muffle the sound as much as necessary, 10-mm-thick insulation is required, which means high consumption of material and a correspondingly great use of fuel as well as large space requirements.
Also known as state of the art are mats which are wound around pipes and molded parts and which consist of an inner foam layer, lead foil, and an external PVC foil. Such a mat serves to muffle structure- and airborne sound. However, it is also comparatively thick and requires a great effort during installation and repair.